For many decorative applications, it is desirable for plastic parts to have a so-called "class A" surface finish which is a very smooth, lustrous, decorative surface. Post-molding buffing and polishing of the parts can be eliminated if the articles could be molded in such a manner as to provide the class A surface on the article as it leaves the mold. One way to achieve this is to provide a smooth, polished mold surface against which the article is molded.
It is known to produce substantially cast-to-size molds having molding cavities therein conforming substantially to the finished dimensions of the article to be molded. Such molds can be made by simply casting the mold material against a pattern conforming substantially to the finished dimensions of the article to be molded, and thereafter removing the pattern so as to leave a mold cavity available to receive plastic injected thereinto. Typically, such molds will be made in at least two halves with an appropriate parting line therebetween for removal of the pattern as well as the molded plastic article. This cast-to-size approach to mold-making eliminates much of the expense that otherwise occurs from machining the mold parts out of blocks of metal.
It is well known to use certain zinc alloys containing copper and aluminum for casting such cast-to-size molds. A common such alloy contains about 10% to about 11% copper, about 3% to about 4% aluminum, about 0.35% to about 0.55% magnesium, up to 0.06 iron and the balance zinc. This material shrinks somewhat on cooling, and as a result, develops some microporosity throughout the structure. When the molding surface is finished, the finishing process removes some of the surface material and continually exposes some of the sub-surface pores which are often as large as 0.002 inches in diameter. This porosity prevents such molds from forming "class A" finishes on the as-molded parts since. In this regard, under normal operating temperatures and pressures encountered in injection molding, the plastic is forced into the surface pores and creates a roughened surface on the molded article.
It is also old and known to prepare cast-to-size molds by starting with a pattern (e.g., plaster) conforming substantially to the finished dimensions of the plastic part to be molded, coating it with a release agent, thermal spraying (e.g., wire arc) about 0.03 inches to about 0.04 inches of metal (typically zinc) onto the pattern to form a metal shell thereover, casting highly-filled tool epoxy resin about the metal shell to back up and reinforce the shell, and thereafter curing the epoxy resin. Examples of such tool epoxies are described in Arkins et al U.S. Pat. No. 4,732,962 and Wang et al U.S. Pat. No. 4,920,161 which are assigned to the assignee of the present invention. Such molds, however, have been unacceptable for producing as-molded, articles having class A surfaces because the sprayed metal shell is quite porous, and accordingly presents substantially the same problems as discussed above in connection with the aforesaid zinc alloy molds.
Finally, it has heretofore been proposed to electroplate the molding surfaces of the aforesaid cast-to-size molds to provide the surface finish needed to mold class A finishes. Single and multiple layers of electroplated metal have both been proposed. In one such proposal, for example, a single layer of nickel is used atop the zinc. In another proposal, a layer of silver is first electrodeposited directly onto the zinc to block the diffusion of a subsequently deposited copper into the zinc. A layer of copper is subsequently electrodeposited atop the silver, and finally a hard layer of nickel and/or chromium is electrodeposited atop the copper. Unfortunately, electroplating alone can fill only the shallowest of the surface pores, and a pocked surface can result if the pores are not completely filled. Moreover, even strong "leveling" electrodeposits, such as copper, tend to cover over the larger pores rather than completely fill them because the metal deposits rapidly at the pore opening and either inhibits deposition deeper within the pore or forms a cap covering over and closing off the pore. This cap seals off the pore and often traps the plating solution therein which, in turn, can subsequently either ( 1) corrode the deposit, (2) upon heating, vaporize and erupt through the coating, or (3) expand enough upon heating to raise/lift the coating causing blisters to occur on the molding surface. Finally, if electrolyte is not trapped within the pore, the cap is susceptible to deformation or rupture inwardly of the pore incident to applied pressure of the molten plastic being injected into the mold cavity. Some of these problems might be eliminated if the electroplating were performed very slowly over a prolonged period of time, but such an approach is not commercially practiced. A quicker and more reliable method for preparing the mold's surface for class-A molding is desired.
It is an object of the present invention to provide a process for manufacturing a substantially cast-to-size mold for directly molding class A surfaces onto molded plastic articles including so filling the otherwise porous molding surface of a cast-to-size mold so as to permit the electrodeposition of a smooth metal coating thereon for molding class A surfaces thereagainst and directly onto the as-molded article. This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.